7 tredir

Contents of this section

tredir is one of TERM's most powerful utilities, allowing most important network services to be performed over a TERM link. Before we explain how to use tredir, it is necessary to give some background on network services. We have talked about network services before, but we have not said exactly what they are. Services are just that - services that are provided by the network. Examples of services include telnet, which provides logins between machines, the File Transfer Protocol, ftp, which transfers files between machines, and smtp, the Simple Mail Transfer Protocol, which is used whenever you send electronic mail. Each network service has a port number associated with it. The mapping of port numbers to services is given in the file '/etc/services'. This file should be the same on all internet-connected machines.

How are these services invoked? Each networked machine runs a daemon called inetd, which listens for attempts to connect to the network ports. These requests can come from either the network or the local machine. A network service is obtained by connecting to a particular inetd port. When a network request is made, inetd knows exactly which service is involved by the port number the request is made on. If inetd is configured to do so, it provides the relevant service to the requesting connection. inetd's configuration is given by the file '/etc/inetd.conf', which has a list of the services that inetd provides. For more information, see the man pages for inetd and inetd.conf.

You can communicate directly with network services by using telnet (n.b. not termtelnet). For example, to talk to the sendmail, (or smtp) daemon on machine machine_name, you can do a telnet machine_name smtp, or telnet machine_name 25, (since 25 is the number assigned to smtp in '/etc/services'). You should get a polite greeting from the daemon on the remote machine. This is a very useful trick for debugging network problems and checking ports redirected with tredir (see below).

Tredir works very much like inetd. It runs in the background as a daemon, listening to the network ports, waiting for a request. When a request for a service is made, instead of providing that service, as inetd does, tredir forwards the request over the TERM link to the remote TERM, which makes the request over the network, returning the result back over the link to the local client. Tredir can forward the request to any machine on the network, but by default sends it to the machine at the other end of the TERM link. Tredir redirects TCP (Transmission Control Protocol) network services over the TERM link.

The common command format of tredir is:

  tredir [this_computer:]port [that_computer:]port

An example should make this clear. Let us redirect a local port to the telnet port on the remote machine. To do this we would do

  tredir 2023 23

Now, anyone who connects to port 2023 on the local machine will be redirected to port 23 (telnet) on the remote machine. Here is an example session; the local machine is mymachine.modem.home and the remote machine is netsun.

  $ tredir 2023 23 
  Redirecting 2023 to 23 
  $ telnet localhost 2023 
  Connected to mymachine.modem.home 
  Escape character is '^]'. 

  SunOS UNIX (netsun)

This example is actually quite useful. If I were instead to do the tredir on netsun. I could then telnet in to mymachine from the network simply by connecting to the redirected port on the networked machine (using telnet) - i.e. telnet netsun 2023.

The general principle in using tredir is to redirect the desired service to a machine on the network. Our next example will allow us to read news on the local machine over our TERM link from a news server on the network. News is provided by the nntp service, port number 119. All decent news readers allow you to specify what port number they will use, either via a configuration file or an environment variable. Let us specify this local port to be 2119. Now, let us say that our news server is news.domain.org. We will redirect port 2119 to port 119 on news.domain.org; we will then tell our news reading software that the nntp server is located at port 2119 on the local host. Since this will depend on the news reader that you use, I will just test the link with telnet instead of firing up a news reader:

  $ tredir 2119 news.domain.org:119
  Redirecting 2119 to news.domain.org:119 
  $ telnet localhost 2119 
  Connected to mymachine.modem.home. 
  Escape character is '^]'. 
  200 news.domain.org InterNetNews NNRP server INN 1.4 07-Dec-41 ready 
  (posting ok). 

If you can get this far, all you have to do is configure your news reader to be able to read news via TERM. (n.b., if you read news like this, be sure that in all your posts you set a Reply-To: header to an network email address that you can be reached at, otherwise people who want to get in touch with you will be sending mail to whatever (wrong) data your news reader puts in the From: header).

7.1 tredir can bite!

The astute reader, after reading the last example will be wondering why port 2119 was redirected to port 119 - since news readers default to port 119, why could I not do a tredir 119 news.domain.org:119 and skip the news reader configuration? The answer is that all ports numbered less than 1024 are ``reserved ports'', and only the superuser can listen to them. If one is willing to take a security risk and make tredir an SUID program, or run tredir as root, then one can redirect reserved ports and avoid the hassle of renaming services.

Another problem with using reserved ports is that inetd is often already listening to these ports, and only one program at a time can listen to a port. In order to use such a port, you must change 'inetd.conf' so that inetd no longer listens to the port you want to redirect. This is most easily done by commenting out the line with the offending service by putting a # character at the beginning of the line. The superuser must then send inetd a HUP signal (kill -1 inetd-pid) to get it to reread its configuration.

7.2 Stupid tredir tricks

In this section we will describe some of the more common uses for tredir. We have already described how to redirect nntp and telnet services; here we will give some more complicated examples.

X windows

In a previous section, we described how to get an X client running on the network to open a window on your home machine using txconn. The same technique could be used on your home machine to display a client on the machine at the remote end of your TERM link. But how does one display an X client on a network machine that is not the remote end? The answer lies in knowing that X uses a particular network service just like the other programs we have been discussing. An X server listens for a network request on a port whose number is given by the formula port = 6000 + display number, e.g. an X server managing screen 0 on a machine would listen to port 6000, if it were managing screen 2, it would listen to port 6002. When you set your DISPLAY environment variable to xmachine:n, your X clients will try to connect to port 6000 +n on xmachine.

We can use this to trick X clients on your local machine to open up windows on remote displays. Let us say I want to open up an xterm, running on my local machine, on display 0 of machine xmachine, which is running some place on the network. I first pick a local display number, say 2 (do not use 0, since that is what your local X server will be using). I will map this display to display 0 on xmachine. In terms of ports, this means I want to redirect the local port 6002 to the remote port 6000. I do the following

  $ tredir 6002 xmachine:6000 
  $ setenv DISPLAY localhost:2
  $ xterm 

This should open up an xterm on machine xmachine. Note that I set the DISPLAY to localhost:2. This is because X clients will sometimes use unix domain sockets instead of internet domain sockets, at their own option, when connecting to a local display, if DISPLAY is set to :2. localhost:2 says to use a TCP connection.

Note that as far as xmachine is concerned, the X request is coming from the machine on the remote end of your TERM link (remotemachine) - so if you need to authorize the connection, you should either do an xhost + remotemachine on xmachine or use xauth to update the '.Xauthority' file on your local machine for display number 2, using the key from xmachine.

Again, to speed up X connections, you can use the program sxpc, which includes an explanation of how to use tredir to establish the link and authorize it using xauth.

Mail with TERM

Well, you asked for it. Electronic mail has the justifiable reputation of being one of the most difficult things to get working right on a UNIX system. To really get TERM working correctly with mail means that you have to understand how mail works, which is beyond the scope of this document. To learn more about mail, you should consult a book on UNIX system administration and/or the comp.mail.misc FAQ, available for anonymous ftp on rtfm.mit.edu:pub/usenet/comp.mail.misc. There are also currently two packages available for anonymous ftp on sunsite.unc.edu that will help you get mail running under TERM - they are term.mailerd+smail by Byron A. Jeff and the BCRMailHandlerXXX by Bill C. Riemers.

That being said, we will give a thumbnail description of how mail works. There are two parts to getting mail running, sending messages and receiving messages. We will begin with sending messages from your local box to the network.

There are two classes of mail programs. The first is the mail user agent (MUA). MUA's help you read, compose and send messages. Examples of MUA's are elm, pine, Mail and vm. MUA's do not really do any networking; they just put the messages together - the real work of sending mail is done by the second class of mail programs, the mail transfer agents (MTA's). These are invoked by the MUA's. They take the message, decide where to send it by looking at the address, and then actually deliver it over the network.

The two most common MTA's on Linux systems are sendmail and smail. The basic idea is to get your MTA to connect to another MTA running on a machine on the net that will know what to do with your message. This is done by redirecting a local port to the smtp port on the net machine. You then have to tell you MTA to take any message it does not know what to do with, and send it out over the redirected port on your local machine to the MTA on the remote machine, which will then route your message to its correct destination.

How do we do this using smail? We first redirect a port to the smtp port on the network mail machine (mailhost):

tredir XXXX mailhost:25

here XXXX is the port number that the smail on the localhost will connect to (note that I have to give this port a name in my '/etc/services' to get smail to recognize it). Smail has several configuration files that usually live in '/usr/local/lib/smail'. The ones we care about are 'config', 'routers' and 'transports'. Note that I am assuming you have already got smail configured correctly for local mail - delivery to files and pipes and such things. Again, consult the documentation if you have not.

In the file 'config', we put the following definition:


localhost is the machine that smail connects to when it does not know what to do with a message.

In 'routers' we put

  path = localhost 

In 'transports' we put

  termsmtp:        driver=tcpsmtp, 
        append_header="From: YOUR_NET_ADDRESS", 
        -max_addrs, -max_chars; 

In the above, the header lines change the From header in all your outgoing mail to the address, YOUR_NET_ADDRESS, which is the network address you want mail sent to. If more than one user is going to be using your TERM link, you will have to do something more fancy, like keep a database of local user's network addresses and insert these in the From: headers.

The service line is the name of the local port number that you have redirected to the smtp port on the network connected machine. In my version of smail, I cannot just set this to a number, I have to set it to a name, like ``foo'', and then define ``foo'' in my '/etc/services' to be the number of my redirected port. If you use a SUID tredir and just redirect the smtp port (25), you do not need to define this.

This should be enough to get you going. If you decide to use sendmail, the principles are the same but the details differ. Ronald Florence (ron@mlfarm.com) told me that the stock Sun sendmail will not send multiple queued messages over a redirected port; BSD sendmail 8.6.9 works fine. He made the following changes to '/etc/sendmail.cf' to get it working with TERM. In his case, the default sendmail port (25) is used for SMTP traffic over a local ethernet so Internet mail is forwarded to a redirected TCP port.

  #Create the termsmtp mailer, which sends mail via a re-directed TCP port  
  Mtermsmtp,P=[TCP], F=mDFMuCXe, S=22, R=22, A=TCP $h PORTNUMBER

Here, PORTNUMBER is the number of the redirected port on the local machine. This should be an unused port over 2000. We next tell sendmail which machine to connect to, and set termsmtp as the default mailer.

  # major relay mailer
  # major relay host: use the $M mailer to send mail to other domains

Here HOSTNAME is the name of your local host (does localhost work?). The last entry goes under Rule 0 to forward Internet mail.

  # Pass other valid names up the ladder to our forwarder
  R$*<@$*.$+>$*         $#$M    $@$R $:$1<@$2.$3>$4     user@any.domain

When the TERM connection is established to the Internet host, run the following commands on the local machine.

  tredir PORTNUMBER internet.host:25
  /usr/lib/sendmail -q

We now turn to receiving electronic mail using TERM. We will assume that mail is sent to your account on the network machine mailhost. The simplest solution is to just use trsh or termtelnet to log on to mailhost and read your mail on there. However, it is also possible to have your mail automatically downloaded to your local machine. One way to do this is to use the Post Office Protocol, (POP). POP was designed for exactly this purpose: to deliver mail to machines that have intermittent network connections. To use POP, you must have a POP server installed on mailhost. Assuming that you do, you can then use a POP client to download your mail every few minutes. This is done, as you might expect, using tredir. The POP service is 110 (note that there is an older protocol, POP-2, which uses port 109; in this document we describe POP-3, which is the latest version of POP). There are several POP clients available. One, written in the script language perl is pop-perl-1.X, written by William Perry and maintained by myself - it can be found on sunsite.unc.edu:pub/Linux/system/Mail.

To use POP, you redirect a local port to port 110 on mailhost and configure your client to retrieve your mail from localhost using the local port. As an example, we will assume that there is a POP server running on mailhost. We will redirect the local port 2110, and fire up the pop-perl client:

  $ tredir 2110 mailhost:110
  Redirecting 2110 to mailhost:110
  $ pop 
  Username: bill 
  Password: <enter your password for mailhost>
  Pop Host: name of local 
  Pop Port: 2110 
  Starting popmail daemon for bill 

If you do not have a POP server available, the BCRMailHandler package has a program to download your mail over a TERM link to your local machine. I have not used it, but anyone who has is welcome to comment. You can also use the term.mailerd+smail package for this purpose. Alas, both BCRMailHandler and the term.mailerd.smail package do not work anymore with TERM versions 2.0.0 and higher.

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